1. Field of the Invention
This invention relates generally to a transformer, and more particularly to a transformer avoiding undesired radiation.
2. Description of the Prior Art
A prior art transformer used in an inverter will be described with reference to FIG. 1. In FIG. 1, 1 designates an AC power source. The input AC voltage from the AC power source 1 is supplied to a rectifying circuit 2, the rectified output from which is supplied to a series connection of capacitors 11 and 12. A series connection between the collector of a transistor 21 and the emitter of a transistor 22 is connected in parallel to the series connection of the capacitors 11 and 12. Control windings 31 and 32 of an input transformer are connected between the base and the emitter of the transistors 21 and 22, respectively. The emitter of the transistor 21 and the collector of the transistor 22 are connected together to the connection point between the capacitors 11 and 12 through a current feedback winding 33 of the transformer 30 and an input winding 41 of an input transformer 40. A voltage feedback winding 43 of the transformer 40 is connected through a resistor 9 to a voltage feedback winding 34 of the transformer 30, and an output winding 42 of the transformer 40 is connected to a rectifying circuit 4 which is connected to a smoothing circuit 5. In FIG. 1, 6 designates output terminals led out from the smoothing circuit 5; 30a the saturable core of the transformer 30; and 40a the ordinary magnetic core of the transformer 40, respectively.
With the above prior art inverter, when the transistors 21 and 22 are made ON and OFF alternately, a rectangular waveform AC current flows through the input winding 41 of the transformer 40. Thus, desired DC current can be delivered to the output terminals 6. At this time, the AC current flowing through the input winding 41 of the transformer 40 also flows in the winding 33 of the transformer 30, so that current feedback is carried out. The AC voltage induced at the winding 43 of the transformer 40 at this time is supplied to the winding 34 through the resistor 9 to achieve the voltage feedback. Thus, the ON and OFF of the transistors 21 and 22 are repeated.
With such a prior art inverter, if its oscillation frequency is selected high, for example, 20 KHz, the power source transformer 40 can be made compact and light in weight, and accordingly, the inverter itself can be made compact and light in weight.
In general, a power source transformer for a main amplifier used in an audio instrument and the like becomes very large in size. Due to this large size of the power source transformer, the main amplifier is restricted in its size and design and also associated parts are restricted in arrangement. However, with the inverter described above, its size can be made compact and also its shape can be selected freely, so that the main amplifier using the above inverter is not restricted in size and design and also its associated parts are not restricted in arrangement.
Since the above inverter is driven with the rectangular waveform signal, undesired radiations are produced. This will be described in detail. The equivalent circuit of the above inverter in view point of AC can be shown in FIG. 2A. Since the oscillation is carried out by the transistors 21, 22 and the input transformer 30, these are shown in FIG. 2A as an oscillator 3. The output transformer 40 is connected to the oscillator 3. In this case, there is formed a coupling capacity 40' between the input and output windings 41 and 42 of the transformer 40. Thus, the equivalent circuit of the inverter shown in FIG. 1 can be redrawn as in FIG. 2B in view of the coupling capacitor 40'. Further, the output winding 42 is selected the same in potential as that of the chassis (not shown) in view of AC, and the inverter is housed in a shield case 7, as shown in FIG. 2C. Thus, the inverter is connected to the chassis through a capacitor 71 formed between the inverter and the shield case 7.
On the other hand, the input terminal 1 is different from the output terminal 6 in potential in AC point of view, it is impossible to connect the input terminal 1 to the chassis. As a result, the input terminal 1 is floated from the chassis in electrical point of view.
Under such a condition, the rectangular oscillator 3 carries out its rectangular oscillation, so that the input terminal 1 acts as an antenna and hence electric noise waves (undesired radiations) caused by the rectangular waveform voltages are radiated outside through the shield case to badly affect the other circuits.